prop + more hp = more speed @ same rpm?

[Oriole]

Hi everyone,

I was just wondering if you added more hp to your engine, would the same rpm give you anymore speed or just help more for your holeshot?

Reason asking is, I have a 16' 4.3l 2 barrel alpha1 (rated @ 175hp), maybe my pitot speedometer is not working correctly but I just switched from an aluminum black max 19 pitch, to a laser SS 23 pitch, and at the same wot rpm seems I only gained a few mph with the bigger prop. actually wot on the laser is 4700 @ 36mph on the speedo and I can get to 4900 with the 19 pitch @ 32mph. operating range for my motor is 4400-4800
I have to say it definetly feels much faster than 30+ mph, I was also suprised only losing 200 rpms with that big of step. I do have a stupid hydrofoil on there that came with the boat, that I see got to trade off for smart tabs as soon as I can. Boat runs great, just don't understand why I'm not going faster? This is traveling both ways up and down the lake so currents and winds aren't affecting me that much.

 

[kenmyfam]

Re: prop + more hp = more speed @ same rpm?

Hi everyone,

I was just wondering if you added more hp to your engine, would the same rpm give you anymore speed or just help more for your holeshot?

Reason asking is, I have a 16' 4.3l 2 barrel alpha1 (rated @ 175hp), maybe my pitot speedometer is not working correctly but I just switched from an aluminum black max 19 pitch, to a laser SS 23 pitch, and at the same wot rpm seems I only gained a few mph with the bigger prop. actually wot on the laser is 4700 @ 36mph on the speedo and I can get to 4900 with the 19 pitch @ 32mph. operating range for my motor is 4400-4800
I have to say it definetly feels much faster than 30+ mph, I was also suprised only losing 200 rpms with that big of step. I do have a stupid hydrofoil on there that came with the boat, that I see got to trade off for smart tabs as soon as I can. Boat runs great, just don't understand why I'm not going faster? This is traveling both ways up and down the lake so currents and winds aren't affecting me that much.

You should GPS verify the speeds to get good accuracy.

 

[Oriole]

Re: prop + more hp = more speed @ same rpm?

I know that would be the way to go, but I do not know anyone nor myself that has one, and can't afford one right now, my best way I can do it I see, is have one of my friends drive right next to me to compare speedometers.
What is a decent cheap gps/handheld/or fishfinder that can tell me my speed?

 

[Dhadley]

Re: prop + more hp = more speed @ same rpm?

You're not comparing apples to apples as far as the props go. You're also leaving out one very important factor in your formula - drag.

 

[Frank Acampora]

Re: prop + more hp = more speed @ same rpm?

Given the same gear ratio and the same WOT RPM, a 4 inch pitch increase will only give an approximate 9 MPH theoretical increase in speed not counting slip. So, given the reduced RPM seen with the 23 pitch prop, what you are seeing speedwise (4 MPH gain with the Laser) in actuality is close.

 

[Oriole]

Re: prop + more hp = more speed @ same rpm?

I am still going to compare my speedometer next to someone else, but, how would I get my speed up closer to 50mph? bigger prop? which would start bringing rpm down so then I would need more engine power to get that back up right?
+< 24 pitch on a v6 typical though for top speed?

 

[Oriole]

Re: prop + more hp = more speed @ same rpm?

I am still going to compare my speedometer next to someone else, but, how would I get my speed up closer to 50mph? bigger prop? which would start bringing rpm down so then I would need more engine power to get that back up right?
+< 24 pitch on a v6 typical though for top speed?
I hear of a lot of v6 engines hitting the 50mph mark

 

[superpop]

Re: prop + more hp = more speed @ same rpm?

I would not go higher in pitch past the 23P. You are going to start bogging your motor down and that can cook the motor. The best thing to do is figure out what prop lets the motor get to within the recommended RPM range for the motor. Short of that you could put a 4 barrel carb on for more speed. HP and Torque are somewhat related and a 4 barrel carb is going to give you a bit more of both. Plus just sticking a higher pitch prop on the boat is going to effect your hole shot as well. Do you have a compelling reason to go 50MPH in a 16 foot boat? You can probably prop to this goal but your hole shot and mid range speed is going to suck.

 

[steelespike]

Re: prop + more hp = more speed @ same rpm?

175 hp in a 16ft would seem to me should crowd 50.
Certainly a strong low 40s.Heck a 3.0 in a 18foot will usually do around 40. What is the weight of the boat?
Were you running alone? Deep V or relatively fast bottom?Bottom good and clean?Motor in excellent tune? Is the Throttle opening all the way?
How is the compression.How is the hole shot?

 

[Oriole]

Re: prop + more hp = more speed @ same rpm?

was just me (180lbs) and a buddy (195lbs) very light gear, 2 fishin poles, 2 tackle boxes, empty cooler for fish (no beer) and 1/4 tank of gas. Motor runs great, no issues with it at all, hole shot is decent, could be better with a smaller prop, the 19 pitch definetly got me out of the hole better, I noticed the stainless 23 pitch has square vent holes in it, will plugging these help on my boat that has thru transom exhaust?
Not sure on the weight of the boat, bottom is clean, it is a trailer queen, never docked overnight, throttle has been checked, does open all the way, motor was rebuilt 3 yrs ago. I blew through the vacuum line from the dash speedometer to the back, was kinda tough, so blew through with air compressor @ 40 psi, goes through fine. haven't had it back on the water though.

Here is a picture of the boat -

 

[steelespike]

Re: prop + more hp = more speed @ same rpm?

Without the through hub exhaust the vents won't do much.you might as well plug them.Perhaps your speed is faster than indicated.Perhaps the rpm is wrong, reading high.If you look around you can find a GPS for about $100
after rebate.Megellen is one.Did a little checking a 175 outboard pushes a 20ft 2,480lb tritoon at 42 mph,a 2300lb19 ft boat at 43 and 23ft 2,600 lb at 40All dry weights.Granted a different setup but I think you can see something isn't right.

 

[superpop]

Re: prop + more hp = more speed @ same rpm?

Did you disconnect the pitot before blowing 40PSI through the rubber line. If you did not then you probably blew out the pitot. Make sure also that the Pitot is not loose or being pushed up when you are under way and that is is fully in the down position. The spoiler on the back should also be good for another 10MPH at least.

 

[steelespike]

Re: prop + more hp = more speed @ same rpm?

Yes getting rid of foil would be good for a speed increase.

 

[Oriole]

Re: prop + more hp = more speed @ same rpm?

yes, I blew compressed air from the speedometer side, I use tyraps as hose clamps on both ends (speedometer and pitot), the pitot is a year old, and it holds down positively solid, pretty tough to knock it up.

Weather permitting tomorrow, I will check it out tomorrow, I just climbed underneath and polished the bottom hull, looked clean but rubbing my hand down it kinda "stuck" to it, so now its super smooth.

As soon as my hours pick back up at work( been at 32hrs for way too long) I will pick up some smart tabs and a gps, but looking at the back of my transom, I'm not sure if I have room to put the tabs on with the swim decks on.

 

[Tail_Gunner]

Re: prop + more hp = more speed @ same rpm?

A laser is a good prop...but probably not a good match for a I/O..

http://forums.iboats.com/showthread.php?t=240777&highlight=laser+22+pitch


Reason asking is, I have a 16' 4.3l 2 barrel alpha1 (rated @ 175hp), maybe my pitot speedometer is not working correctly but I just switched from an aluminum black max 19 pitch, to a laser SS 23 pitch, and at the same wot rpm seems I only gained a few mph with the bigger prop. actually wot on the laser is 4700 @ 36mph on the speedo and I can get to 4900 with the 19 pitch @ 32mph. operating range for my motor is 4400-4800


The prop is simply not hooking up as Ken said light bass boat..small thin blades

 

[TpaPaul813]

Re: prop + more hp = more speed @ same rpm?

I have a 3.0 Sea Ray 18' and it tops out @ 40..

 

[Oriole]

Re: prop + more hp = more speed @ same rpm?

So I got a gps yesterday, took the boat out after checking over the motor, compression on all 6 cylinders was between 150-160 psi, spark plugs looked great with a light coffee cream color, checked timing, was @ 8*, even waxed the bottom hull the other day. Put the 23 pitch ss laser on and watched the speedometer and gps, they both were in agreement. Topped out at 39mph @ 4700 rpm's with just me and a buddy together weight of about 385 lbs and a 1/8th of a tank of gas.
I would like to get the boat weighed just for a better comparison with other 4.3l boats, but damn, it is only a 16' closed bow v hull. What prop would be a better match for me? This 23 pitch I think does have to much slip, especially on a hole shot. How in the hell are you other guys getting in mid 40's almost 50 with a V6 or even the 4 banger.

 

[n2ostroker]

Re: prop + more hp = more speed @ same rpm?

A laser is a good prop...but probably not a good match for a I/O..

http://forums.iboats.com/showthread.php?t=240777&highlight=laser+22+pitch

The Laser II is a great prop. But not the even pitches. The even pitches are a 13.5" diameter. The odd pitches like has are 13 3/4" and a lot of goes are going really fast with them in 18-22' boats with a lot more than 175hp. It's basically a mini Mirage. I have a 23 Mirage plus sitting here next to my 23 Laser II and they are identical except the amount of blade area.


Edit: meant to say the evens were designed for outboards which is why the smaller diameter. The odds were designed for moderate->higher hp i/o in smaller lighter boats.

 

[Oriole]

Re: prop + more hp = more speed @ same rpm?

What I don't understand is my boat should be fairly light, and I can crank the motor up to the upper operating rpm wot. So where is my speed at? Could it be the prop is really slipping that bad? What other prop will work for a boat like mine? My hull is very similar to something like a checkmate

 

[MikDee]

Re: prop + more hp = more speed @ same rpm?

Lets see, according to my simple formula:

4700rpm divided by (1.65 gear ratio, just guessing here?) = 2848 rpm X 22 (23 -1 pitch)= 62.66 mph, minus 10% = 56.39mph, minus 20% = 50.12mph, minus 30% = 43.86mph, That's it your prop appears to be causing over 30% slip Unless, your tach is lying 10-15% slip is about normal for an efficient combo.